There has been an ongoing trend in the information technology industry to execute software programs more quickly. For example, there are various conventional advancements that provide for increased execution speed of software programs. One technique for increasing execution speed of a program is called parallelism. Parallelism is the practice of executing or performing multiple things simultaneously. Parallelism can be implemented on multiple levels, from executing multiple instructions at the same time, to executing multiple threads at the same time, to executing multiple programs at the same time, and so on.
Instruction Level Parallelism or ILP is parallelism at the lowest level and involves executing multiple instructions simultaneously. Processors that exploit ILP are typically called multiple-issue processors, meaning they can issue multiple instructions in a single clock cycle to the various functional units on the processor chip.
There are different types of conventional multiple-issue processors. One type of multiple-issue processor is a superscalar processor in which a sequential list of program instructions are dynamically scheduled. A respective processor determines which instructions can be executed on the same clock cycle, and sends them out to their respective functional units to be executed. This type of multi-issue processor is called an in-order-issue processor since issuance of instructions is performed in the same sequential order as the program sequence, but issued instructions may complete at different times (e.g., short instructions requiring fewer cycles may complete before longer ones requiring more cycles).
Another type of multi-issue processor is called a VLIW (Very Large Instruction Width) processor. A VLIW processor depends on a compiler to do all the work of instruction reordering and the processor executes the instructions that the compiler provides as fast as possible according to the compiler-determined order. Other types of multi-issue processors issue out of order instructions, meaning the instruction issue order is not be the same order as the order of instructions as they appear in the program.
Conventional techniques for executing instructions using ILP can utilize look-ahead techniques to find a larger amount of instructions that can execute in parallel within an instruction window. Looking-ahead often involves determining which instructions might depend upon others during execution for such things as shared variables, shared memory, interference conditions, and the like. When scheduling, a handler associated with the processor detects a group of instructions that do not interfere or depend on each other. The processor can then issue execution of these instructions in parallel thus conserving processor cycles and resulting in faster execution of the program. As will be discussed, certain embodiments herein are related to thread level parallelism (e.g., TLP).
Thread-level parallelism (TLP) is the parallelism inherent in an application that runs multiple threads at once. This type of parallel processing can be sometimes be found in applications written for commercial servers such as databases. By running many threads at once, applications are able to tolerate the high amounts of I/O and memory system latency their workloads can incur—while one thread is delayed waiting for a memory or disk access, other threads can do useful work.